1. Field of the Invention
The present invention relates to an insulating layer applied to a thin film transistor substrate, and more particularly, to a composition of an organic insulating layer.
2. Discussion of the Related Art
A thin film transistor is widely used as a switching element of a display device such as a liquid crystal display (LCD) or an organic light emitting display (OLED).
The thin film transistor comprises a gate electrode, an active layer, a source electrode and a drain electrode. Then, a passivation layer is formed on the thin film transistor.
Hereinafter, a related art thin film transistor substrate will be described with reference to the accompanying drawings.
FIG. 1 is a cross sectional view of a related art thin film transistor substrate. As shown in FIG. 1, a gate electrode 20 is formed on a substrate 10, and a gate insulating layer 30 is formed on the gate electrode 20.
Then, a semiconductor layer 40 is formed on the gate insulating layer 30, and a source electrode 52 and a drain electrode 54 are formed on the semiconductor layer 40, wherein the source electrode 52 and the drain electrode 54 are positioned opposite to each other.
A passivation layer 60 is formed on the source electrode 52 and the drain electrode 54, and a pixel electrode 70 is formed on the passivation layer 60. In this case, a predetermined contact hole (H) is formed in the passivation layer 60, whereby the pixel electrode 70 is electrically connected with the drain electrode 54 through the contact hole (H).
As mentioned above, the passivation layer 60 is formed on the thin film transistor, to thereby protect the thin film transistor. Generally, the passivation layer 60 for protecting the thin film transistor may be formed of an organic insulating layer.
The passivation layer 60 is patterned to have the predetermined contact hole (H). A process for patterning the passivation layer 60 comprises an exposing process using a predetermined mask, and a development process using a predetermined developing solution. That is, the exposing process is carried out through the use of predetermined mask, whereby a chemical reaction occurs in an exposed region of the passivation layer 60. Under these conditions, the development process is carried out, whereby the exposed region is removed by the developing solution due to different material properties between the exposed region and non-exposed region, thereby forming the contact hole (H).
It should be possible to pattern the contact hole (H) in the passivation layer 60. Thus, the organic insulating layer used for the passivation layer 60 comprises a photosensitizer capable of making the chemical reaction to the exposing process (to a light such as UV).
In case of the related art, the photosensitizer included in the organic insulating layer is typically formed of a photoactive compound (PAC).
However, if using the photoactive compound (PAC) as the photosensitizer, it has the following disadvantages.
First, in order to form a fine pattern, that is, in order to form the contact hole (H) with a precise pattern, when using the photoactive compound (PAC) as the photosensitizer, it is necessary to additionally carry out a baking process for applying a predetermined heat between the exposing process and the development process.
That is, since the photoactive compound (PAC) has low reactivity to the exposing process, the baking process is additionally carried out after the exposing process, whereby the entire regions are clearly divided into the region to be removed by the following development process, and the region not to be removed.
Also, the photoactive compound (PAC) has a yellow color. Thus, if using the photoactive compound (PAC), it might lower a light transmittance of a display device and cause a problem of color-coordinates shift.
Meanwhile, when the photoactive compound (PAC) is irradiated with ultraviolet rays (UV), its yellow color is changed into a white color so that it is possible to improve the light transmittance. However, for this, an entire-surface exposing process has to be additionally carried out. Further, even though the yellow color is changed into the white color by the entire-surface exposing process, it is difficult to obtain the high-level light transmittance.
As described above, if using the photoactive compound (PAC) as the photosensitizer, the light transmittance of the display device is deteriorated, and an entire manufacturing process is complicated due to the additional processes such as the baking process and entire-surface exposing process.